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      <h3><a href="../../../index.htm"><img height="86" width="277" alt="C++ Boost" src="../../../boost.png" border="0"></a></h3>
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      <h1 align="center">Serialization</h1>
      <h2 align="center">Class Serialization Traits</h2>
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<hr>
<dl class="page-index">
  <dt><a href="#version">Version</a>
  <dt><a href="#level">Implementation Level</a>
  <dt><a href="#tracking">Object Tracking</a>
  <dt><a href="#export">Export Key</a>
  <dt><a href="#abstract">Abstract</a>
  <dt><a href="#typeinfo">Type Information Implementation</a>
  <dt><a href="#wrappers">Wrappers</a>
  <dt><a href="#bitwise">Bitwise Serialization</a>
  <dt><a href="#templates">Template Serialization Traits</a>
  <dt><a href="#compiletime_messages">Compile Time Warnings and Errors</a>
</dl>
Serialization of data depends on the type of the data.  For example, for
primitive types such as <code style="white-space: normal">int</code>, it wouldn't make sense to save
a version number in the archive. Likewise, for a data type that is never
serialized through a pointer, it would (almost) never make sense to track
the address of objects saved to/loaded from the archive as it will never
be saved/loaded more than once in any case.  Details of
serialization for a particular data type will vary depending on the
type, the way it is used and specifications of the programmer. 
<p>
One can alter the manner in which a particular data type is serialized
by specifying one or more <strong>class serialization traits</strong>.
It is not generally necessary for the programmer to explictly assign
traits to his classes as there are default values for all traits.
If the default values are not appropriate they can be assigned by the programmer.
A template is used to associate a typename with a constant.  For example
see <a href="../../../boost/serialization/version.hpp" target="version_hpp">
version.hpp</a>.  
<h3><a name="version">Version</a></h3>
This header file includes the following code:

<pre><code>
namespace boost { 
namespace serialization {
template&lt;class T&gt;
struct version
{
    BOOST_STATIC_CONSTANT(unsigned int, value = 0);
};
} // namespace serialization
} // namespace boost
</code></pre>

For any class <code style="white-space: normal">T</code>, The default definition 
of <code style="white-space: normal">boost::serialization::version&lt;T&gt;::value</code> is 0. 
If we want to assign a value of 2 as the version for class <code style="white-space: normal">my_class</code>
we specialize the version template:
<pre><code>
namespace boost { 
namespace serialization {
struct version&lt;my_class&gt;
{
    BOOST_STATIC_CONSTANT(unsigned int, value = 2);
};
} // namespace serialization
} // namespace boost
</code></pre>
Now whenever the version number for class <code style="white-space: normal">my_class</code> is required,
the value 2 will be returned rather than the default value of 0.
<p>
To diminish typing and enhance readability, a macro is defined
so that instead of the above, we could write:
<pre><code>
BOOST_CLASS_VERSION(my_class, 2)
</code></pre>
which expands to the code above.

<h3><a name="level">Implementation Level</a></h3>
In the same manner as the above, the "level" of implementation of serialization is
specified.  The header file <a href="../../../boost/serialization/level.hpp" 
target="level_hpp">level.hpp</a> defines the following.
<pre><code>
// names for each level
enum level_type
{
    // Don't serialize this type. An attempt to do so should
    // invoke a compile time assertion.
    not_serializable = 0,
    // write/read this type directly to the archive. In this case
    // serialization code won't be called.  This is the default
    // case for fundamental types.  It presumes a member function or
    // template in the archive class that can handle this type.
    // there is no runtime overhead associated reading/writing
    // instances of this level
    primitive_type = 1,
    // Serialize the objects of this type using the objects "serialize"
    // function or template. This permits values to be written/read
    // to/from archives but includes no class or version information. 
    object_serializable = 2,
    ///////////////////////////////////////////////////////////////////
    // once an object is serialized at one of the above levels, the
    // corresponding archives cannot be read if the implementation level
    // for the archive object is changed.  
    ///////////////////////////////////////////////////////////////////
    // Add class information to the archive.  Class information includes
    // implementation level, class version and class name if available.
    object_class_info = 3,
};
</code></pre>
Using a macro defined in <code style="white-space: normal">level.hpp</code> we can specify
that <code style="white-space: normal">my_class</code> should be serialized along with its version number:
<pre><code>
BOOST_CLASS_IMPLEMENTATION(my_class, boost::serialization::object_class_info)
</code></pre>
If implementation level is not explicitly assigned, the system uses
a default according to the following rules.
<ul>
  <li>if the data type is <code style="white-space: normal">volatile</code> 
assign <code style="white-space: normal">not_serializable</code>
  <li>else if it's an enum or fundamental type assign <code style="white-space: normal">primitive_type</code>
  <li>else assign <code style="white-space: normal">object_class_info</code>
</ul>
That is, for most user defined types, objects will be serialized along with
class version information. This will permit one to maintain backward
compatibility with archives which contain previous versions.  However, with this 
ability comes a small runtime cost.  For types whose definition will "never" 
change, efficiency can be gained by specifying <code style="white-space: normal">object_serializable</code> 
to override the default setting of <code style="white-space: normal">object_class_info</code>.  
For example, 
this has been done for the  
<a href="../../../boost/serialization/binary_object.hpp" target="binary_object_hpp">
binary_object wrapper</a>

<h3><a name="tracking">Object Tracking</a></h3>
Depending on the way a type is used, it may be necessary or convenient to
track the address of objects saved and loaded.  For example, this is generally
necessary while serializing objects through a pointer in order to be sure
that multiple identical objects are not created when an archive is loaded.
This "tracking behavior" is controlled by the type trait defined in the header
file <a href="../../../boost/serialization/tracking.hpp" target="tracking_hpp">tracking.hpp</a>
which defines the following:
<pre><code>
// names for each tracking level
enum tracking_type
{
    // never track this type
    track_never = 0,
    // track objects of this type if the object is serialized through a 
    // pointer.
    track_selectively = 1,
    // always track this type
    track_always = 2
};
</code></pre>
A corresponding macro is defined so that we can use:
<pre><code>
BOOST_CLASS_TRACKING(my_class, boost::serialization::track_never)
</code></pre>
Default tracking traits are:
<ul>
  <li>For primitive, <code style="white-space: normal">track_never</code>.
  <li>For pointers, <code style="white-space: normal">track_never</code>.
  That is, addresses of addresses are not tracked by default.
  <li>All current serialization wrappers such as <code style="white-space: normal">boost::serialization::nvp</code>,
  <code style="white-space: normal">track_never</code>.
  <li>For all other types, <code style="white-space: normal">track_selectively</code>.
  That is addresses of serialized objects are tracked if and only if
  one or more of the following is true:
  <ul>
  <li>an object of this type is anywhere in the program serialized
  through a pointer.
  <li>the class is explicitly "exported" - see below.
  <li>the class is explicitly "registered" in the archive
  </ul>
</ul>

<p>
The default behavior is almost always the most convenient one.  However,
there a few cases where it would be desirable to override the
default.  One case is that of a virtual base class. In a diamond
heritance structure with a virtual base class, object tracking
will prevent redundant save/load invocations.  So here is one
case where it might be convenient to override the default tracking
trait. <i>(Note: in a future version the default will be reimplemented
to automatically track classes used as virtual bases).</i> This 
situation is demonstrated by 
<a href="../test/test_diamond.cpp" target="test_diamond_cpp">test_diamond.cpp</a>
included with the library.

<h3><a name="export">Export Key</a></h3>

When serializing a derived class through a virtual base class pointer,
two issues may arise.
<ul>
<li> The code in the derived class might never be explicitly
referred to.  Such code will never be instantiated.
<p>
This is addressed by invoking 
<code style="white-space: normal">BOOST_CLASS_EXPORT_IMPLEMENT(T)</code>
in the file which defines (implements) the class T.
This ensures that code for the derived class T will
be explicity instantiated.
<li> There needs to be some sort of identifier which can
be used to select the code to be invoked when the object
is loaded.
Standard C++ does implement <code style="white-space: normal">typeid()</code> which can be
used to return a unique string for the class.  This is not entirely
statisfactory for our purposes for the following reasons:
<ul>
  <li>There is no guarantee that the string is the same across platforms.
  This would then fail to support portable archives.
  <li>In using code modules from various sources, classes may have
  to be wrapped in different namespaces in different programs.
  <li>There might be classes locally defined in different code modules
  that have the same name. 
  <li>There might be classes with different names that we want to
  consider equivalent for purposes of serialization.
</ul>
<p>
So in the serialization library, this is addressed by invoking
<code style="white-space: normal">BOOST_CLASS_EXPORT_KEY2(my_class, "my_class_external_identifier")</code>
in the header file which declares the class.
In a large majority of applications, the class name works just fine
for the external identifier string so the following short cut is
defined -
<code style="white-space: normal">BOOST_CLASS_EXPORT_KEY(my_class)</code>.
</ul>
For programs which consist of only one module - that is 
programs which do not use DLLS, one can specify
<code style="white-space: normal">BOOST_CLASS_EXPORT(my_class)</code>
or
<code style="white-space: normal">BOOST_CLASS_EXPORT_GUID(my_class, "my_class_external_identifier")</code>
in either the declaration header or definition.  These macros
expand to invocation of both of the macros described above.
<i>(<b>GUID</b> stands for <b>G</b>lobally <b>U</b>nique <b>ID</b>entfier.)</i>
<p>
<i>(<a target="detail" href="special.html#export">Elsewhere</a>
in this manual, the serialization of derived classes is addressed in detail.)</i>
<p>
The header file
<a href="../../../boost/serialization/export.hpp" target="export_hpp">export.hpp</a>
contains all macro definitions described here.
The library will throw a runtime exception if
<ul>
<li> A type not explicitly referred to is not exported.
<li> Serialization code for the same type is instantiated
in more than one module (or DLL).
</ul>

<h3><a name="abstract">Abstract</a></h3>
When serializing an object through a pointer to its base class,
the library needs to determine whether or not the base is abstract
(i.e. has at least one virtual function).  The library uses the
type trait macro <code style="white-space: normal">BOOST_IS_ABSTRACT(T)</code>
to do this.  Not all compilers support this type trait and corresponding
macro.  To address this, the macro <code style="white-space: normal">
BOOST_SERIALIZATION_ASSUME_ABSTRACT(T)</code> has been
implemented to permit one to explicitly indicate that a specified
type is in fact abstract.  This will guarentee that
<code style="white-space: normal">BOOST_IS_ABSTRACT</code>
will return the correct value for all compilers.

<h3><a name="typeinfo">Type Information Implementation</a></h3>
This last trait is also related to the serialization of objects
through a base class pointer.  The implementation of this facility
requires the ability to determine at run time the true type of the
object that a base class pointer points to.  Different serialization
systems do this in different ways.  In our system, the default method
is to use the function <code style="white-space: normal">typeid(...)</code> which is available 
in systems which support <b>RTTI</b> (<b>R</b>un <b>T</b>ime 
<b>T</b>ype <b>I</b>nformation).
This will be satisfactory in almost all cases and most users of this 
library will lose nothing in skipping this section of the manual.
<p>
However, there are some cases where the default type determination
system is not convenient.  Some platforms might not support
RTTI or it may have been disabled in order to speed execution
or for some other reason.  Some applications, E.G. runtime linking
of plug-in modules, can't depend on C++ RTTI to determine the
true derived class.  RTTI only returns the correct type for polymorphic
classes - classes with at least one virtual function.  If any of these
situations applies, one may substitute his own implementation of
<code style="white-space: normal">extended_type_info</code>
<p>
The interface to facilities required to implement serialization is defined in
<a href="../../../boost/serialization/extended_type_info.hpp" 
target="extended_type_info_hpp">extended_type_info.hpp</a>.

Default implementation of these facilities based on <code style="white-space: normal">typeid(...)</code>
is defined in

<a href="../../../boost/serialization/extended_type_info_typeid.hpp" 
target="extended_type_info_typeid_hpp">extended_type_info_typeid.hpp</a>.

An alternative implementation based on exported class identifiers
is defined in
<a href="../../../boost/serialization/extended_type_info_no_rtti.hpp" 
target="extended_type_info_rtti_hpp">extended_type_info_no_rtti.hpp</a>.
<p>
By invoking the macro:
<pre><code>
BOOST_CLASS_TYPE_INFO(
    my_class, 
    extended_type_info_no_rtti&lt;my_class&gt;
)
</code></pre>
we can assign the type information implementation to each class on a case by
case basis.  There is no requirement that all classes in a program use the same 
implementation of <code style="white-space: normal">extended_type_info</code>.  This supports the concept
that serialization of each class is specified "once and for all" in a header
file that can be included in any project without change.
<p>
This is illustrated by the test program
<a href="../test/test_no_rtti.cpp" target="test_no_rtti_cpp">test_no_rtti.cpp</a>.
Other implementations are possible and might be necessary for
certain special cases.

<h3><a name="wrappers">Wrappers</a></h3>
Archives need to treat wrappers differently from other types since, for example,
they usually are non-const objects while output archives require that any
serialized object (with the exception of a wrapper) be const. 

This header file <a href="../../../boost/serialization/wrapper.hpp">wrapper.hpp</a>
includes the following code:

<pre><code>
namespace boost { 
namespace serialization {
template&lt;class T&gt;
struct is_wrapper
 : public mpl::false_
{};
} // namespace serialization
} // namespace boost
</code></pre>

For any class <code style="white-space: normal">T</code>, The default definition 
of <code style="white-space: normal">boost::serialization::is_wrapper&lt;T&gt;::value</code> is thus false.
 
If we want to declare that a class <code style="white-space: normal">my_class</code>
is a wrapper we specialize the version template:
<pre><code>
namespace boost { 
namespace serialization {
struct is_wrapper&lt;my_class&gt;
 : mpl::true_
{};
} // namespace serialization
} // namespace boost
</code></pre>
<p>
To diminish typing and enhance readability, a macro is defined
so that instead of the above, we could write:
<pre><code>
BOOST_CLASS_IS_WRAPPER(my_class)
</code></pre>
which expands to the code above.

<h3><a name="bitwise">Bitwise Serialization</a></h3>
Some simple classes could be serialized just by directly copying all bits
of the class. This is, in particular, the case for POD data types containing
no pointer members, and which are neither versioned nor tracked. Some archives, 
such as non-portable binary archives can make us of this information to 
substantially speed up serialization.

To indicate the possibility of bitwise serialization the type trait defined 
in the header
file <a href="../../../boost/serialization/is_bitwise_serializable.hpp" target="is_bitwise_serializable">is_bitwise_serializable.hpp</a>
is used:
<pre><code>
namespace boost { namespace serialization {
    template<class T>
    struct is_bitwise_serializable
     : public is_arithmetic<T>
    {};
} }
</code></pre>
is used, and can be specialized for other classes. The specialization
is made easy by the corresponding macro:
<pre><code>
BOOST_IS_BITWISE_SERIALIZABLE(my_class)
</code></pre>

<h3><a name="templates">Template Serialization Traits</a></h3>
In some instances it might be convenient to assign serialization traits
to a whole group of classes at once.  Consider, the name-value pair
wrapper 
<pre><code>
template&lt;class T&gt;
struct nvp : public std::pair&lt;const char *, T *&gt;
{
    ...
};
</code></pre>
used by XML archives to associate a name with a data variable of type T.
These data types are never tracked and never versioned.  So one might
want to specify:
<pre><code>
BOOST_CLASS_IMPLEMENTATION(nvp&lt;T&gt;, boost::serialization::level_type::object_serializable)
BOOST_CLASS_TRACKING(nvp&lt;T&gt;, boost::serialization::track_never)
</code></pre>
Examination of the definition of these macros reveals that they won't expand
to sensible code when used with a template argument.  So rather than using the
convenience macros, use the original definitions
<pre><code>
template&lt;class T&gt;
struct implementation_level&lt;nvp&lt;T&gt; &gt;
{
    typedef mpl::integral_c_tag tag;
    typedef mpl::int_&lt;object_serializable&gt; type;
    BOOST_STATIC_CONSTANT(
        int,
        value = implementation_level::type::value
    );
};

// nvp objects are generally created on the stack and are never tracked
template&lt;class T&gt;
struct tracking_level&lt;nvp&lt;T&gt; &gt;
{
    typedef mpl::integral_c_tag tag;
    typedef mpl::int_&lt;track_never&gt; type;
    BOOST_STATIC_CONSTANT(
        int, 
        value = tracking_level::type::value
    );
};
</code></pre>
to assign serialization traits to all classes generated by the template
<code style="white-space: normal">nvp&lt;T&gt;</code>
<p>

Note that it is only possible to use the above method to assign traits to 
templates when using compilers which correctly support Partial Template Specialization.

One's first impulse might be to do something like:

<pre><code>
#ifndef BOOST_NO_TEMPLATE_PARTIAL_SPECIALIZATION
template&lt;class T&gt;
struct implementation_level&lt;nvp&lt;T&gt; &gt;
{
   ... // see above
};

// nvp objects are generally created on the stack and are never tracked
template&lt;class T&gt;
struct tracking_level&lt;nvp&lt;T&gt; &gt;
{
   ... // see above
};
#endif
</code></pre>
This can be problematic when one wants to make his code <strong>and archives</strong>
portable to other platforms.  It means the objects will be serialized differently
depending on the platform used.  This implies that objects saved from one platform
won't be loaded properly on another.  In other words, archives won't be portable.
<p>
This problem is addressed by creating another method of assigning serialization traits
to user classes.  This is illustrated by the serialization for a 
<a target="nvp" href="../../../boost/serialization/nvp.hpp"><strong>name-value</strong> pair</a>.
<p>
Specifically, this entails deriving the template from a special class
<a target="traits" href="../../../boost/serialization/traits.hpp">
<code style="white-space: normal">boost::serialization::traits</code></a> which is specialized for a specific
combination of serialization traits.  
When looking up the serialization traits, the library first checks to see if this class has been
used as a base class. If so, the corresponding traits are used.  Otherwise, the standard defaults
are used. By deriving from a serialization traits class rather than relying upon Partial Template
Specializaton, one can a apply serialization traits to a template and those traits will be
the same across all known platforms.
<p>
The signature for the traits template is:
<pre><code>
template&lt;
    class T,       
    int Level, 
    int Tracking,
    unsigned int Version = 0,
    class ETII = BOOST_SERIALIZATION_DEFAULT_TYPE_INFO(T),
    class IsWrapper = mpl::false_
&gt;
struct traits
</code></pre>
and template parameters should be assigned according to the following table:
<p>
<table border>
<tr><th align=left>parameter</th><th align=left>description</th><th align=left>permitted values</th><th align=left>default value</th></tr>
<tr><td><code>T</code></td><td>target class</td><td>class name<T></td><td>none</td></tr>            
<tr><td><code>Level</code></td><td>implementation level</td><td><code>not_serializable<br>primitive_type<br>object_serializable<br>object_class_info</code></td><td>none</td></tr>  
<tr><td><code>Tracking</code></td><td>tracking level</td><td><code>track_never<br>track_selectivly<br>track_always</code></td><td>none</td></tr>
<tr><td><code>Version</code></td><td><code>class version</td><td>unsigned integer</td><td><code>0</code></td></tr>
<tr><td><code>ETTI</code></td><td><code>type_info</code> implementation</td><td><code>extended_type_info_typeid<br>extended_type_info_no_rtti</code></td><td>default <code>type_info implementation</code></td></tr>
<tr><td><code>IsWrapper</code></td><td><code></code>is the type a wrapper?</td><td><code>mpl::false_<br>mpl::true_</code></td><td><code>mpl::false_</code></td></tr>
</table>

<h3><a name="compiletime_messages">Compile Time Warnings and Errors</a></h3>
Some serialization traits can conflict with other ones.  Sometimes these conflicts
will result in erroneous behavior (E.G. creating of archives which could not be read)
and other times they represent a probable misconception on the part of the
library user which could result in suprising behavior.  To the extent possible,
these conflicts are detected at compile time and errors (BOOST_STATIC_ASSERT) 
or warnings (BOOST_STATIC_WARNING) are generated. They are generated in a
compiler dependent manner which should show a chain of instantiation
to the point where the error/warning is detected.  Without this capability,
it would be very hard to track down errors or unexpected behavior in library
usage.  Here is a list of the conflicts trapped:

<dl>
<dt><h2><a name="object_level">object_level</a> - error</h2></dt>
<dd>
This error traps attempts to serialize types whose
implentation level is set to non_serializable.
</dd>
<dt><h2><a name="object_versioning">object_versioning</a> - error</h2></dt>
<dd>
It's possible that for efficiency reasons, a type can be
assigned a serialization level which doesn't include type information 
in the archive.  This would preclude the assignment
of a new version number to the type.  This error
traps attempts to assign a version number in this case.
This has to be a user error.
</dd>

<dt><h2><a name="object_tracking">object_tracking</a> - warning</h2></dt>
<dd>
The following code will display a message when compiled:

<code style="white-space: normal"><pre>
T t;
ar &lt;&lt; t;
</pre></code>

unless the tracking_level serialization trait is set to "track_never". The following
will compile without problem:

<code style="white-space: normal"><pre>
const T t
ar &lt;&lt; t;
</pre></code>

Likewise, the following code will trap at compile time:

<code style="white-space: normal"><pre>
T * t;
ar >> t;
</pre></code>

if the tracking_level serialization trait is set to "track_never".
<p>

The following case illustrates the function of this message.
It was originally used as an example in the
mailing list by Peter Dimov.

<code style="white-space: normal"><pre>
class construct_from 
{ 
    ... 
}; 

void main(){ 
    ... 
    Y y; 
    construct_from x(y); 
    ar &lt;&lt; x; 
} 
</pre></code>

Suppose that the above message is not displayed and the code is used as is.
<ol>
  <li>this example compiles and executes fine. No tracking is done because 
  construct_from has never been serialized through a pointer. Now some time 
  later, the next programmer(2) comes along and makes an enhancement. He 
  wants the archive to be sort of a log. 

<code style="white-space: normal"><pre>
void main(){ 
    ... 
    Y y; 
    construct_from x(y); 
    ar &lt;&lt; x; 
    ... 
    x.f(); // change x in some way 
   ... 
    ar &lt;&lt; x 
} 
</pre></code>
  <p>
  Again no problem. He gets two different of copies in the archive, each one is different. 
  That is he gets exactly what he expects and is naturally delighted. 
  <p>
  <li>Now sometime later, a third programmer(3) sees construct_from and says - 
  oh cool, just what I need. He writes a function in a totally disjoint 
  module. (The project is so big, he doesn't even realize the existence of 
  the original usage) and writes something like: 

<code style="white-space: normal"><pre>
class K { 
    shared_ptr &lt;construct_from&gt; z; 
    template &lt;class Archive&gt; 
    void serialize(Archive & ar, const unsigned version){ 
        ar &lt;&lt; z; 
    } 
}; 
</pre></code>

  <p>
  He builds and runs the program and tests his new functionality. It works 
  great and he's delighted. 
  <p>
  <li>Things continue smoothly as before.  A month goes by and it's 
  discovered that when loading the archives made in the last month (reading the 
  log). Things don't work. The second log entry is always the same as the 
  first. After a series of very long and increasingly acrimonius email exchanges, 
it's discovered 
  that programmer(3) accidently broke programmer(2)'s code .This is because by 
  serializing via a pointer, the "log" object is now being tracked.  This is because
  the default tracking behavior is "track_selectively".  This means that class
  instances are tracked only if they are serialized through pointers anywhere in
  the program. Now multiple saves from the same address result in only the first one 
  being written to the archive. Subsequent saves only add the address - even though the 
  data might have been changed.  When it comes time to load the data, all instances of the log record show the same data.
  In this way, the behavior of a functioning piece of code is changed due the side
  effect of a change in an otherwise disjoint module.
  Worse yet, the data has been lost and cannot be recovered from the archives.
  People are really upset and disappointed with boost (at least the serialization system).
  <p>
  <li>
  After a lot of investigation, it's discovered what the source of the problem is
  and class construct_from is marked "track_never" by including:
<code style="white-space: normal"><pre>
BOOST_CLASS_TRACKING(construct_from, track_never) 
</pre></code>
  <li>Now everything works again. Or - so it seems.
  <p>
  <li><code style="white-space: normal">shared_ptr&lt;construct_from&gt;</code>
is not going to have a single raw pointer shared amongst the instances. Each loaded 
<code style="white-space: normal">shared_ptr&lt;construct_from&gt;</code> is going to 
have its own distinct raw pointer. This will break 
<code style="white-space: normal">shared_ptr</code> and cause a memory leak.  Again,
The cause of this problem is very far removed from the point of discovery.  It could 
well be that the problem is not even discovered until after the archives are loaded.
Now we not only have a difficult to find and fix program bug, but we have a bunch of
invalid archives and lost data.
</ol>

<p>Now consider what happens when the message is displayed:

<ol>
  <p>
  <li>Right away, the program traps at 
<code style="white-space: normal"><pre>
ar &lt;&lt; x; 
</pre></code>
  <p>
  <li>The programmer curses (another %^&*&* hoop to jump through). He's in a 
  hurry (and who isn't) and would prefer not to <code style="white-space: normal">const_cast</code>
  - because it looks bad.  So he'll just make the following change an move on. 
<code style="white-space: normal"><pre>
Y y; 
const construct_from x(y); 
ar &lt;&lt; x; 
</pre></code>
  <p>
  Things work fine and he moves on. 
  <p>
  <li>Now programer (2) wants to make his change - and again another 
  annoying const issue; 
<code style="white-space: normal"><pre>
Y y; 
const construct_from x(y); 
... 
x.f(); // change x in some way ; compile error f() is not const 
... 
ar &lt;&lt; x 
</pre></code>
  <p>
  He's mildly annoyed now he tries the following: 
  <ul>
    <li>He considers making f() a const - but presumably that shifts the const 
    error to somewhere else. And he doesn't want to fiddle with "his" code to 
    work around a quirk in the serializaition system 
    <p>
    <li>He removes the <code style="white-space: normal">const</code>
    from <code style="white-space: normal">const construct_from</code> above - damn now he 
    gets the trap. If he looks at the comment code where the 
    <code style="white-space: normal">BOOST_STATIC_ASSERT</code>
    occurs, he'll do one of two things 
    <ol>
      <p>
      <li>This is just crazy. Its making my life needlessly difficult and flagging 
      code that is just fine. So I'll fix this with a <code style="white-space: normal">const_cast</code>
      and fire off a complaint to the list and mabe they will fix it. 
      In this case, the story branches off to the previous scenario.
      <p>
      <li>Oh, this trap is suggesting that the default serialization isn't really 
      what I want. Of course in this particular program it doesn't matter. But 
      then the code in the trap can't really evaluate code in other modules (which 
      might not even be written yet). OK, I'll add the following to my 
      construct_from.hpp to solve the problem. 
<code style="white-space: normal"><pre>
BOOST_CLASS_TRACKING(construct_from, track_never) 
</pre></code>
    </ol>
  </ul>
  <p>
  <li>Now programmer (3) comes along and make his change. The behavior of the 
  original (and distant module) remains unchanged because the 
  <code style="white-space: normal">construct_from</code> trait has been set to 
  "track_never" so he should always get copies and the log should be what we expect.
  <p>
  <li>But now he gets another trap - trying to save an object of a 
  class marked "track_never" through a pointer. So he goes back to 
  construct_from.hpp and comments out the 
  <code style="white-space: normal">BOOST_CLASS_TRACKING</code> that 
  was inserted. Now the second trap is avoided, But damn - the first trap is 
  popping up again. Eventually, after some code restructuring, the differing
  requirements of serializating <code style="white-space: normal">construct_from</code>
  are reconciled.
</ol>
Note that in this second scenario
<ul>
  <li>all errors are trapped at compile time.
  <li>no invalid archives are created.
  <li>no data is lost.
  <li>no runtime errors occur.
</ul>

It's true that these messages may sometimes flag code that is currently correct and
that this may be annoying to some programmers.  However, this example illustrates
my view that these messages are useful and that any such annoyance is a small price to
pay to avoid particularly vexing programming errors.

</dd>

<dt><h2><a name="pointer_level">pointer_level</a> - warning</h2></dt>
<dd>
This trap addresses the following situaion when serializing 
a pointer:
<ul>
<li>A type doesn't save class information in the
archive. That is, the serialization trait implementation
level <= object_serializable.
<li>Tracking for this type is set to "track selectively"
in this case, indication that an object is tracked is
not stored in the archive itself - see level == object_serializable.
Since class information is not saved in the archive, the existence
or absence of the operation ar << T * anywhere else in the 
program is used to infer that an object of this type should be tracked.
<p>
A problem arises when a program which reads an archive
includes the operation ar >> T * so that tracking information
will be included in the archive. When a program which
creates the archive doesn't include ar << T it is presumed
that the archive doesn't include tracking information and
the archive will fail to load.  Also the reverse situation could 
trigger a similar problem.
<p>
Though this situation is unlikely for several reasones, 
it is possible - hence this warning. 
</ul>
So if your program traps here, consider changing the 
tracking or implementation level traits - or not
serializing via a pointer.
</dd>

<dt><h2><a name="pointer_tracking">pointer_tracking</a> - warning</h2></dt>
<dd>
Serializing an object of a type marked "track_never" through a pointer
could result in creating more objects than were saved! There are cases
in which a user might really want to do this so we leave it as a warning.
</dd>

<dt><h2><a name="const_loading">const_loading</a> - error</h2></dt>
<dd>
One cannot load data into a "const" object unless it's a
wrapper around some other non-const object.
</dd>
</dl>

<hr>
<p><i>&copy; Copyright <a href="http://www.rrsd.com">Robert Ramey</a> 2002-2004 and Matthias Troyer 2006. 
Distributed under the Boost Software License, Version 1.0. (See
accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
</i></p>
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